Isolation Methods

While different species of animals hybridize rarely, or with extreme difficulty and (quite frankly) luck, plants are the complete opposite. While some plants will not cross with others, many will. Even if you’re growing plants that won’t cross-pollinate, chances are you’re growing more then one variety of a plant that will. So if you’re saving your own seed, and you don’t want the luck of the draw with what will come out next year, you’ll want to take some steps to avoid natural cross-pollination.

While being a home gardener doesn’t afford us the luxury of hundreds of feet (in some cases, kilometres) of distance between our varieties or plants, there are steps that even the smallest-space home gardener can take in order to ensure that cross-pollination doesn’t occur.

It’s important to keep in mind that there may be neighbours near by that are also growing crops that can cross-pollinate with yours.

Distance Isolation
Distance is exactly that – the distance between your different plants, or different varieties of the same plant, to avoid cross-pollination either by wind, animals, or insects. The following is a list of USDA recommended distances between different plant and variety types to ensure seed purity.

Bean (Common, Fava, Lima)

40 metres (1320 ft)

Beet

8 kilometres (5 miles)

Brussels Sprouts

200 metres (660 ft)

Cabbage

200 metres (660 ft)

Cantaloupe

40 metres (1320 ft)

Carrot

40 metres (1320 ft)

Cauliflower

200 metres (660 ft)

Corn

40 metres (1320 ft)

Cucumber

40 metres (1320 ft)

Eggplant

N/A*

Lettuce

N/A*

Pea

N/A*

Pepper (Hot, Sweet)

9 metres (30 ft)

Potato

9 metres (30 ft)

Squash (Summer, Winter, Pumpkin)

40 metres (1320 ft)

Radish

40 metres (1320 ft)

Spinach

8 kilometres (5 miles)

Tomatillo

9 metres (30 ft)

Tomato

9 metres (30 ft)

Turnip

200 metres (660 ft)

Watermelon

40 metres (1320 ft)

*Note: While the USDA doesn’t deem it necessary for any sort of distance, these numbers are based on crops grown in large quantity in fields, rather than inter-plantings normal for the home gardener. I would still suggest a barrier method, especially if different varieties are being grown closely together.

It is important to remember that groups of different plants/varieties are more likely to cross-pollinate when grown in close approximation to each other. Since this is how most home gardeners grow their crops, take this into account, and use a barrier or time isolation method to ensure your seed comes true.

Barrier Isolation
The barrier method is using a physical barrier between your plants or varieties to ensure that wind or pollinators don’t exchange pollen from one plant to the other. It must be noted that having a barrier like a wall or your house in between two different varieties or crops can hamper cross-pollination, it is by no means a sure thing, especially if your distances between plants are relatively close.

Bagging – Bagging is extremely simple, and a good effective way to avoid cross-pollination. I prefer to buy yards of pre-made cheap organza bags from craft supply stores and use them as is. Take these bags, and before the flowers open (important!), tie the bags around the buds. Leave the bag there until you can see fruit forming on the plants. If you tie the bag on after the flowers have opened, or if it is taken off before the flowers die off completely, there’s no way to ensure 100% purity of the seed. This method is best for plants that are self-fertilizing or self-pollinating as no hand-pollination will have to be done on your part. Plants like tomatoes, peppers, and beans are all self-fertilizing. Paper bags can also be used for this purpose, although since they can’t be seen through, you’re constantly having to untie and retie the bag to the flower cluster. Plastic should not be used, as this can often create an environment within the bag that is to hot for fruit to properly set.

A special note on squash – the bagging method is recommended to keep squash varieties pure since they very easily cross (and in fact require pollen from the male flower to be placed inside the female flower). See the seed saving article on squash, melons, and cucumbers here for more information.

Caging – Caging is much the same as bagging, but instead of isolating individual flower clusters, instead a box is built that can be put over the whole plant with a wood or metal skeleton with mesh attached to it (fine screen door netting and tulle are good options for this).

Time Isolation
Time isolation involves extreme organization. It involves timing your plants, either with different maturity rates, or staggered planting, to bloom at different times. That way when pollinators do fly in, they exchange pollen that is either incompatible, or it exchanges only pollen from that plant with its flowers. This is an excellent choice for plants that require pollination to occur from flowers other then its own (like tomatillos), but unfortunately it doesn’t take into account neighbours who may be growing different varieties, or compatible plants with yours.